Vapor degreasing and solvent cleaning with fluorocarbon based solvents have found widespread use in industry for the degreasing and otherwise cleaning of solid surfaces, especially intricate parts and difficult to remove soils.
In its simplest form, vapor degreasing or solvent cleaning consists of exposing a room temperature object to be cleaned to the vapors of a boiling solvent. Vapors condensing on the object provide clean distilled solvent to wash away grease or other contamination. Final evaporation of solvent from the object leaves behind no residue as would be the case where the object is simply washed in liquid solvent.
For difficult to remove soils where elevated temperature is necessary to improve the cleaning action of the solvent, or for large volume assembly line operations where the cleaning of metal parts and assemblies must be done efficiently and quickly, the conventional operation of a vapor degreaser consists of immersing the part to be cleaned in a sump of boiling solvent which removes the bulk of the soil, thereafter immersing the part in a sump containing freshly distilled solvent near room temperature, and finally exposing the part to solvent vapors over the boiling sump which condense on the cleaned part. In addition, the part can also be sprayed with distilled solvent before final rinsing.
Vapor degreasers suitable in the above-described operations are well known in the art. For example, Sherliker et al. in U.S. Pat. No. 3,085,918 disclose such suitable vapor degreasers comprising a boiling sump, a clean sump, a water separator, and other ancillary equipment.
Cold cleaning is another application where a number of solvents are used. In most cold cleaning applications, the soiled part is either immersed in the fluid or wiped with rags or similar objects soaked in solvents and allowed to air dry.
Fluorocarbon solvents, such as trichlorotrifluoroethane, have attained widespread use in recent years as effective, nontoxic, and nonflammable agents useful in degreasing applications and other solvent cleaning applications. Trichlorotrifluoroethane has been found to have satisfactory solvent power for greases, oils, waxes and the like. It has therefore found widespread use for cleaning electric motors, compressors, heavy metal parts, delicate precision metal parts, printed circuit boards, gyroscopes, guidance systems, aerospace and missile hardware, aluminum parts and the like.
Azeotropic or azeotrope-like compositions are desired because they do not fractionate upon boiling. This behavior is desirable because in the previously described vapor degreasing equipment with which these solvents are employed, redistilled material is generated for final rinse-cleaning. Thus, the vapor degreasing system acts as a still. Unless the solvent composition exhibits a constant boiling point, i.e., is azeotrope-like, fractionation will occur and undesirable solvent distribution may act to upset the cleaning and safety of processing. Preferential evaporation of the more volatile components of the solvent mixtures, which would be the case if they were not azeotrope-like, would result in mixtures with changed compositions which may have less desirable properties, such as lower solvency towards soils, less inertness towards metal, plastic or elastomer components, and increased flammability and toxicity. The art has looked towards azeotrope or azeotrope-like compositions including the desired fluorocarbon components such as trichlorotrifluoroethane which include components which contribute additionally desired characteristics, such as polar functionality, increased solvency power, and stabilizers.
The art is continually seeking new fluorocarbon, hydrofluorocarbon, and hydrochlorofluorocarbon based azeotrope-like mixtures which offer alternatives for new and special applications for vapor degreasing and other cleaning applications. Currently, of particular interest, are fluorocarbon, hydrofluorocarbon, and hydrochlorofluorocarbon based azeotrope-like mixtures with minimal or no chlorine which are considered to be stratospherically safe substitutes for presently used chlorofluorocarbons (CFCs). The latter are suspected of causing environmental problems in connection with the earth's protective ozone layer. Mathematical models have substantiated that hydrofluorocarbons, such as 2-trifluoromethyl-1,1,1,2-tetrafluorobutane (known in the art as HFC-467), will not adversely affect atmospheric chemistry, being negligible contributors to ozone depletion and to green-house global warming in comparison to chlorofluorocarbons such as 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113).
European Publication 431,458 published Jun. 12, 1991 teaches a mixture of 1,1,2,3,4,4-hexafluorobutane and ethanol. U.S. Pat. No. 5,023,010 teaches an azeotropic mixture of 1,1,1,2,3,3-hexafluoro-3-methoxypropane and methanol. U.S. Pat. No. 5,035,830 teaches an azeotropic mixture of hexafluoropropylene/ethylene cyclic dimer and methanol or ethanol. U.S. Pat. No. 5,064,559 teaches an azeotropic mixture of 1,1,1,2,3,4,4,5,5,5-decafluoropentane and methanol or ethanol. U.S. Pat. No. 5,073,291 teaches an azeotrope-type mixture of 1,4-dihydroperfluorobutane and methanol.
U.S. Pat. Nos. 5,073,288 and 5,073,290 teach binary azeotrope-like compositions of 1,1,1,2,2,3,5,5,5-nonafluoro-4-trifluoromethylpentane or 1,1,1,2,2,5,5,5-octafluoro-4-trifluoromethylpentane and methanol or ethanol.